Abstract

In the context of rising greenhouse gas concentrations, and the potential\nfeedbacks between climate and the carbon cycle, there is an urgent\nneed to monitor the exchanges of carbon between the atmosphere and\nboth the ocean and the land surfaces. In the so-called top-down approach,\nthe surface fluxes of CO2 are inverted from the observed spatial\nand temporal concentration gradients. The concentrations of CO2 are\nmeasured in-situ at a number of surface stations unevenly distributed\nover the Earth while several satellite missions may be used to provide\na dense and better-distributed set of observations to complement\nthis network. In this paper, we compare the ability of different\nCO2 concentration observing systems to constrain surface fluxes.\nThe various systems are based on realistic scenarios of sampling\nand precision for satellite and in-situ measurements. It is shown\nthat satellite measurements based on the differential absorption\ntechnique (such as those of SCIAMACHY, GOSAT or OCO) provide more\ninformation than the thermal infrared observations (such as those\nof AIRS or IASI). The OCO observations will provide significantly\nbetter information than those of GOSAT. A CO2 monitoring mission\nbased on an active (lidar) technique could potentially provide an\neven better constraint. This constraint can also be realized with\nthe very dense surface network that could be built with the same\nfunding as that of the active satellite mission. Despite the large\nuncertainty reductions on the surface fluxes that may be expected\nfrom these various observing systems, these reductions are still\ninsufficient to reach the highly demanding requirements for the monitoring\nof anthropogenic emissions of CO2 or the oceanic fluxes at a spatial\nscale smaller than that of oceanic basins. The scientific objective\nof these observing system should therefore focus on the fluxes linked\nto vegetation and land ecosystem dynamics.